Electron Sextets as Optically Addressable Molecular Qubits: Triplet Carbenes

There is a growing demand in quantum information science and sensing for electron spin purification and readout via a spin-optical interface. This technique, known as optically detected magnetic resonance (ODMR), has been applied on diamond-NV centres and transition metal complexes. Metal-free counterparts of these optically addressable spin qubits promise to be cheaper, more sustainable colour centres with prolonged polarisation lifetimes. However, progress has been hindered by the low ODMR signals of carbon-based π-diradicals, partly due to the lack of a ground singlet-to-triplet intersystem crossing (ISC). In this work, we propose to explore organic systems that are even more electron-deficient: electron sextets. Using triplet carbenes as an example, we illustrate how the ground singlet-triplet gap can be widened beyond thermal energy with the associated singlet-to-triplet ISC made available by vibronic effects. Through careful molecular engineering, this ISC can occur at a rate similar to and with an opposite spin selectivity from the excited-state ISC well-established in π-diradicals, unlocking a new ODMR pathway with potential signal gains. Persistent triplet carbenes is a renascent field with multiple stable molecules being isolated in the past five years. To motivate further development of its emissive properties, we illustrate our design in two realistic carbene candidates that incorporate existing strategies for carbene stabilisation. Ultimately, we believe that a new realm of quantum materials can be uncovered by expanding our scope towards stable electron sextets.